The invention relates to illuminating equipment, particularly for photographic color-enlarging apparatus with a color-mixing compartment.
In known photographic color enlargers, illuminating equipment is employed wherein color filters in the three subtractive primary colors are continuously inserted into the beam of rays of a light source emitting white light, the color composition of the copying light being changed depending on the depth of insertion of the individual color filters into the beam. Since the individual color filters usually intersect only a portion of the beam of rays, the color distribution over its cross section is not homogeneous, so that suitable color mixing means must be provided for homogenizing the color distribution of the copying light before the light reaches the original to be copied. Such color-mixing means comprise mixing compartments with reflecting inner surfaces, which are provided for an intensification of the mixing effect and have light-diffusing plates, usually at the ends where the light enters and/or emerges from. The light-diffusing plates at the light emerging side moreover affect the distribution of brightness. This is achieved by means of light-diffusion plates with variously distributed degrees of transmission, which in the center of the light-emerging surface should be smaller than in its peripheral areas, in order that the peripheral areas may appear brighter and thus an even light distribution is obtained and/or lens vignetting is counteracted. Light-diffusing plates are employed for this purpose whose thickness decreases toward the edges.
A good color mixture in these known apparatus is achieved only by relatively large color-mixing compartments, something which involves a double drawback of high light losses on the one hand and of influencing the structural size of the respective illuminating equipment by the dimensions of the color-mixing compartment. Color-mixing compartments can be shortened with the same mixing effect as compared to known designs if diffusing plates with increased diffusing effect are simultaneously employed which, however, again causes very high light losses. In addition, with a shortening of the color-mixing compartment, it must be taken into consideration that the light distribution at the light-emerging surface will deteriorate, which can be counteracted only by the use of a diffusion plate whose thickness is very much greater in the center of the light-emerging surface than towards the edges.
The object of the invention is to provide an illuminating device of the above-mentioned type, which makes possible a shortening of the color-mixing compartment as well as the use of an even diffusing plate at its light-emerging surface without thereby deteriorating the color-mixing conditions and the light distribution as compared to known devices.